Streptococcus pneumoniae is a leading cause of acute otitis media (OM)and responsible for over 1 million infant deaths each year from pneumonia, meningitis, and bacteremia. With the emergence and dissemination of antibiotic resistant pneumococcal strains, coupled to changing patterns of virulence and the inadequacy of available vaccines, medical management of these "pneumococcal diseases" has become increasingly complex and costly. S. pneumoniae is highly adapted to and colonizes asymptomatically the mucosal surface of the nasopharynx. The ability of S. pneumoniae to bind to the host middle ear mucosa (MEM) is a critical step to initiate OM. The mechanisms that control S. pneumoniae either to colonize the nasopharynx or to invade to the MEM causing OM are unclear. S. pneumoniae undergoes spontaneous intrastrain variations in colony morphology that affect multiple cell-surface structures, pathogen-host interaction and virulence. Transparent (T) variants are more capable of colonizing the nasopharynx, whereas the opaque (O) variants show increased virulence during systemic infections. Environmental factors (e.g., oxygen and pressure) and several genes associated with the phenotypic variations were identified. Middle ear (ME)is a sterile and air-filled cavity and its volume of the contained gas determines its pressure, which is regulated by mucosal-blood diffuse and Eustachian tube (ET)function. ET is a critical passagewayfrom the nasopharynx into the ME. The dynamic interactions between S. pneumoniae and host ET function-ME environment, which trigger pathogen to breach host defense, translocate and invade to the ME leading to infection have yet to be established. We hypothesize that ET dysfunction and associated ME gas/pressure dysregulation induce the predominant selection of the virulent phenotypes and enhance pneumococcal virulence, which cause acute OM. We have three specific aims: Aim 1 is to identify pneumococcal phenotypic variations during ME gas/pressure dysregulations in vivo (a rat model of acute OM constructed with ET obstruction followed by ME inoculation of either T or O variants, then by ventilation tube insertion) and in vitro (human ME epithelial cell line exposed to selective pressures and T or O variants). Aim 2 is to assess gene expression coding for virulence factors that were previously tested either in vivo or in vitro to be associated with pneumococcal phase variations and virulence. Aim 3 is to link host response to phenotypic variations and virulence by evaluation of the selected proinflammatory mediators in the MEM.Identification of pneumococcal variants and associated virulence genes that are preferentially transcribed during the phenotypic switch under the altered ME environment is central to our understanding of how pneumococcus mounts a successful ME infection. The information gained from this study holds a promise for clinical molecular diagnostics and therapy by targeted gene interruptions of virulence.